Heterochrony Evolutionary Shifts in Development

Termites are essentially the Peter Pans of the insect world—many individuals never grow up. Most colony members are juveniles whose progressive development has been suspended. Even mature adult termites exhibit numerous juvenile traits when compared to adult cockroaches, the phylogenetically appropriate reference group (Nalepa and Bandi, 2000). Termites therefore may be described as paedomorphic, a term denoting descendent species that resemble earlier ontogenetic stages of ancestral species (Reilly, 1994). The physical resemblance of termites and young cockroaches is indisputable, and is most obvious in the bodily proportions, the thin cuticle, and a short pronotum that leaves the head exposed. Cleveland et al. (1934) and Huber (1976) both noted the resemblance of early instars of Cryptocercus to larger termite species, with the major difference being the more rapid movement and longer antennae of Cryptocercus (Fig. 9.5). One advantage that termites gain by remaining suspended in this thin-skinned morphological state is the avoidance of a heavy nitrogenous (Table 4.5) investment in cuticle typical of older developmental stages of their cockroach relatives.

Cockroaches that are paedomorphic display a variety of termite-like characters such as thinning of the cuticle, eye reduction, and decrease in the size of the pronotal

Fig. 9.5 First instar of Cryptocercus punctulatus. Photo by C.A. Nalepa.

shield (e.g., Nocticola australiensis—Roth, 1988). These cockroaches are often wingless, but when wings are retained they can resemble those of termite alates. In Noc-ticola babindaensis and the genus Alluaudellina (= Allu-audella),the forewings and hindwings are nearly the same length, they considerably exceed the tip of the abdomen, both sets are membranous, and they have a reduced venation and anal lobe (Shelford, 1910a; Roth, 1988).

The expression of altered developmental timing in termites is not limited to morphological characters. It includes aspects of both behavior and physiology that are more characteristic of the juvenile rather than the adult stages of their non-eusocial relatives. Just as maturation of the body became truncated during paedomorphic evolution in the termite lineage, so did many features of behavioral and physiological development. Elsewhere in this chapter we noted several behaviors that are common to termites and cockroach taxa, including burrowing, building, substrate manipulation, trail following, and vi-brational alarm behavior. There are additional behaviors crucial to termite social cohesion shared only with the early developmental stages of cockroaches (Nalepa and Bandi, 2000). In most cockroach species, young nymphs have the strongest grouping tendencies, and in some, early instars are the only stages that aggregate (Chapter 8). Early cockroach instars often display the most pronounced kin recognition (Evans and Breed, 1984), the most intense cannibalism (Wharton et al., 1967; Roth, 1981a), and the most frequent coprophagy (Nalepa and Bandi, 2000).Young Periplaneta nymphs affix fecal pellets to the substrate more often than do older stages (Dele-porte, 1988). Antennal cropping is displayed in nymphs of two cockroach species, and it is only young developmental stages of Cryptocercus that allogroom (Seelinger and Seelinger, 1983). All of these behaviors are standard elements of the termite behavioral repertoire.

Many behaviors shared by termites and young cockroaches relate to food intake. Termites also resemble cockroach juveniles in aspects of digestive physiology and dietary requirements (Nalepa and Bandi, 2000). More so than older stages, early instars of cockroaches rely on conspecific food and ingested microbial protein to fuel growth, and are dependent on the metabolic contributions of microbial symbionts in both the gut and fat body for normal development. As termites evolved, they elaborated on this food-sharing, microbe-dependent mode instead of shifting to a more adult nutritional physiology during ontogenetic growth.

Caste control in termites also may be rooted in the developmental physiology of young cockroaches (Nalepa and Bandi, 2000). It is the early cockroach instars that are most susceptible to developmental perturbations related to nutrition, injury, and group effects (Woodruff, 1938; Seamans and Woodruff, 1939; Holbrook and Schal, 1998). Moreover, these stimuli are extrinsically controllable and may allow for manipulation of individual development by fellow colony members (Nalepa and Bandi, 2000).

In sum, a large number of the juvenile characters of their cockroach ancestors were co-opted by termites in the course of their evolution, and these were integral in the cascade of adaptations and co-adaptations that resulted in the highly derived, eusocial taxon it is today. Heterochrony is known to provide a basis for rapid divergence and speciation,because integrated character sets are typically under a system of hierarchical control (Gould, 1977; Futuyma, 1986). Simple changes in regulatory genes, then, can result in rapid, drastic phenotypic changes (Futuyma, 1986; Stanley, 1998).

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